The proposal defines a mentored career development plan for a K01 candidate with a long-standing interest in hematopoietic stem cell (HSC) research and specialty training in veterinary oncology. The goal of the award is to provide a structured program that will facilitate Dr. Xuan Pan's transition into a veterinary scientist with independent NIH funding resources. Dr. Emery Bresnick will serve as the mentor for this K01 award. His expertise in stem cell biology and his extensive experience in training young scientists will be invaluable for this K01 award. In addition to Dr. Bresnick' mentorship, this K01 award will be supported by a group of top-tier stem cell researchers including Dr. James Thomson, Dr. Jing Zhang and Dr. Chengcheng Zhang. Dr. Pan graduated from the VMD-PhD combined degree program at the University of Pennsylvania. She was recently offered a position as Assistant Professor of Medical Oncology at the University of Wisconsin-Madison School of Veterinary Medicine. As her career goal is to conduct high-level science in the field of hematology, this K01 award will allow her to transit from a veterinary clinician to an independent veterinary scientist. The focus of this project is to investigate Polycomb Group (PcG) protein Yin Yang 1 (YY1) functions in hematopoietic stem cell (HSC) self-renewal and differentiation. YY1 is a unique and crucial mammalian PcG protein, as it recruits other PcG members to specific chromosomal sites, resulting in functionally important histone modification patterns. Dr. Pan's prior data showed that YY1 expression induced an expansion of long-term hematopoietic stem cells (LT-HSCs) in bone marrow-transplanted mice. In this proposal, Dr. Pan will use gain-of-function (Aim 1a) and loss-of-function (Aim 1b) studies to dissect specific mechanistic questions regarding how YY1 controls HSC self-renewal and differentiation. She will use an innovative mutant (YY1 ?REPO), which ablates all YY1 PcG specific functions; to study the underlying mechanisms of YY1 regulated LT-HSC enrichment (Aim 1c). Mechanistically, she will assess whether YY1 represses Cdkn2a and Cdkn1a, two well-established PcG regulatory loci in HSCs, by recruiting other PcG proteins to these loci, and inducing repressive histone modifications in HSCs (Aim 2). Understanding the functional significance and underlying mechanisms of YY1 in HSC biology may provide new avenues for possible therapeutic intervention in hematologic malignancies.